Investigation of the Laminate Structure of Lumbar Disc Anulus Fibrosus

The structure of the lumbar disc anulus fibrosus was investigated using a layer-by-layer peeling technique and microscopic examination of various cut surfaces. Anulus specimens from spines of two different age groups and from two levels, L2-3 and L4-5, were examined. The vertebra-disc-vertebra units were subjected to intentional controlled dehydration to enhance the visual contrast between the white opaque fiber bundles and the translucent ground substance. The variations of the anulus structure with circumferential and radial locations were studied. The following principal structural features were quantified: 1) the anulus, excluding the transition zone, consists of 15 to 25 distinct layers, depending on the circumferential location, the spine level, and the specimen age; 2) in any 20 degrees circumferential sector, nearly half of the layers terminate or originate, thereby causing local laminate irregularities; 3) there are two identifiable mechanisms of layer interruption at these irregularities; 4) the thickness of individual layers varies both circumferentially and radially and increases markedly with age; and 5) the number of fiber bundles over the total height of the disc varies from 20 to 62, with an average interbundle spacing of 0.22 mm.